The specific aim of this project is to obtain a better understanding of the mechanism by which eukaryotic cell differentiation is controlled, especially the role played by RNA processing. The work will be focused on the switching phenomenon which produces the two different mRNAs for two immunoglobulin heavy Mu chains, a membrane associated form (Mum) and a secreted form (Mus), during B lymphocyte cell differentiation. The recent studies on DNA sequencing of the mouse immunoglobulin heavy Mu chain gene constant region have suggested that both of the two mRNAs coding for Mum and Mus chains could be produced from a long precursor by alternate RNA processing pathways. The questions to be asked are whether RNA processing is one of the pivotal steps for such differential gene expression in B lymphocyte development, and what mechanism is concerned with the switching from Mum to Mus mRNA synthesis. As a model gene to study the switching mechanism, we have constructed a chimeric gene plasmid which carries the SV40 promoter sequences and a part of the constant region of mouse immunoglobulin heavy Mu chain gene. The mechanism will be studied using this gene chimera in the Xenopus (African clawed frog) oocyte microinjection system, in the simian cell infection system, and in an in vitro transcription and processing coupled assay system. The factors which control the switching will be isolated, and the biochemical features of their action in the switching mechanism will be studied. The anticipated results of this research project will add significantly to our fundamental knowledges of cell differentiation and gene expression in eukaryotes, and will directly contribute to an understanding of human developmental diseases.